The present invention concerns, first, a device for stirring up gas flowing through a duct and, second, a method of using the device.
Devices for stirring up flowing gas are needed for processing the flue gases that occur when coal, refuse, sludge, and other materials are burned. Such gases contain certain undesirable but unavoidable pollutants, which are removed in downstream scrubbers. Among these pollutants are nitrogen oxides, which can be reduced by adding a reduction agent to the gas.
The oxide-reduction agent in some known versions of the method are mixtures of ammonia and water, added in the form of a mist to gas through pneumatic nozzles. The mist evaporates rapidly in the high heat, and the liquid phase converts to a gas phase. The accordingly enriched gas is forwarded to a catalyzer, where the oxides are broken down. Success here demands matching the concentrations of each reaction partner. If too little reduction agent is added at a particular point, the oxides will decompose incompletely. This is unsatisfactory when the amounts of emissions over time are to be kept low. The addition of too much reduction agent at a particular point on the other hand will generally leave too much of it in the gas, leading to impermissible emissions of that material. The method can only be carried out satisfactorily when the gas is thoroughly mixed with the oxide-reduction agent. The elimination of local temperature differences that derive from irregular loads on the heat exchanger or from the operation of a burner integrated into the duct is also to be recommended. Since the rate of reaction is temperature-dependent, local irregularities in the mean gas-temperature curve over time will limit how much material the reactor can actually separate while reducing the oxides. Variations in temperature over time, however, will be compensated to some extent by the thermally inert mass of the catalyzer material.
Local differences in concentration and temperature are eliminated at the state of the art with static mixers. Introducing the oxide-reduction agent through intersecting-pipe gratings installed in the reduction agent into the path of the gas are known. These registers, or distributors, have many sites for the agent to emerge from. The gas is blended with the agent by turbulence downstream of the pipes. How thoroughly the substances are combined is technically limited by the number of pipes. Furthermore, a reduction-agent introduction grating of intersecting pipes entails a considerable and undesirable loss of pressure.
Satisfactory mixture can also be achieved by rotating some components of the main stream, with the axis of rotation extending along the main axis of flow. A known static mixer accommodates a mixing structure in the form of a surface coiled around the main stream axis and accordingly curved. A series of such structures will ensure a satisfactory mixture. There are drawbacks to this approach, however. Their curvature complicates the design and takes up space. Furthermore, each structure extends all the way across the path of the gas.
Another mixer of this genus employs a structure that exploits the wake deriving from agitation plates mounted against the wall of the duct. These plates are approximately trapezoidal, with their base secured to the wall. The three exposed edges are washed all around by the gas. The structures slope along the main direction of flow and are secured by webs in the constriction between them and the wall, where the flow is released, that is. The structures generate two opposing eddies with velocity components normal to the main direction of flow. The paired eddies intensify the mixture in the gas phase. Using several such structures is supposed to ensure satisfactory mixture. A drawback is the relatively long edges of the structures resting against the wall of the duct.
Other static mixers are described in German A 4 123 161. Here, one cross-section of the duct is divided into several rectangular fields, each accommodating a trapezoidal baffle that slopes toward the main direction of flow.
A generic device that mixes several streams of gas together or adds a liquid coolant to a flowing gas is known from German C 2 911 873, German U 8 219 268, and European Patent 0 673 726. This device employs flat insertion structures in the form of symmetrical surfaces. Their edges are washed free on all sides by the fluids being combined. The structures slope at an acute angle into the flowing gas such as to generate a detachment eddy, which the documents call a forward-edge eddy, at the forward edge. This eddy also includes velocity components at an angle to the main stream, intensifying the mixing process. The structures in this known device are circular, elliptical, oval, parabolic, rhomboidal, or triangular. They can be contoured in cross-section or have bent edges or a V-shaped cross-section.
There are drawbacks to this genus of static mixers. The free all-around washing of the structures' edges necessitates a separate support (cf. German U 8 219 268). The shape of the structures allows the flowing gas to induce non-stationary forces that express themselves as vibrations. The supports that secure the structures must be designed to accommodate the mechanical stress occasioned by the vibrations. The supports usually have to be large, with high moments of resistance. The weight of the supports is a serious drawback in that the technology usually requires them to be positioned high enough up inside the reactor to reduce the oxides. This requirement in turn is detrimental to the static design and assembly of the overall reactor.